The invention pertains to a method of delaying the set time of cement and to the compositions produced therefrom. More specifically, the set time of the cement composition is increased through the use of an acidic retarder slurry.
In many instances, it is important to retard or delay the setting time of cement mixtures. For example, increased setting time is desirable in cases where the cement slurry is mixed and then transported to a remote work site. Increased work time or delayed setting is also important in those situations in which the cement is poured and must be troweled or otherwise formed into a desired structural shape.
The prior art has taught the use of coal fly ash in cementitious applications where high calcium containing ash is utilized. These ashes can be used in combination with both Portland cement and retardants or accelerators, to retard or accelerate the set time of the cement. Variations in setting time in cement compositions comprising fly ash pozzolans and Portland cement have been shown to be achieved by changing the order of introduction of an activator (e.g., citric acid or potassium hydroxide). Additionally, premixing of specific chemicals (e.g., citric acid and fly ash) also has been shown to have an effect on the setting times of cement compositions.
Although the prior art discloses that the rate of hardening of cement can be controlled by the use of retarding or accelerating additives, it has failed to teach the role that pH can provide in retarding or delaying the setting time.
In accordance with the invention, the set time of hydraulic cement mixtures of fly ash pozzolans, specifically Class C and Class F, and Portland cement can be delayed by controlling the pH of a retarder slurry that is preferably used to mix with the dry components of the composition. The pH of the retarder slurry is controlled by balancing the salts therein such that the retarder slurry is acidic, or having a pH less than 7. Slower setting mortar and concrete compositions can be prepared with this hydraulic cement while the compositions can still achieve high strengths over time.
With respect to the fly ash component of the cement composition, exemplary fly ashes include a Class xe2x80x9cFxe2x80x9d and xe2x80x9cCxe2x80x9d combination of pozzolan that meets the requirements of ASTM C-618. This specification limits the loss-on-ignition (LOI) content of fly ash pozzolan to less than 6%. The LOI value of fly ash is generally equal to the percent by weight of the unburned carbon content of the fly ash. This carbon content can vary from as little as about 0.5% up to 20% or more of the weight of the total fly ash product. However, for pozzolanic activity, a good quality fly ash should contain less than 1% carbon but, in any case, no more than about 4% carbon.
Generally, balance of non-lignite Class F fly ash vis-a-vis the more alkaline Class C ash can be controlled so as to control the pH of the dry components, to delay or to retard the setting of the cement mix. Lignitic Class F fly ash contains significant amounts of CaO, and thus its use in the instant invention is not preferred.
Preferably, Portland Cement (ASTM C-150) maybe used in the instant cement compositions as the hydraulic cement component. An ASTM Type I Portland Cement can be mentioned as exemplary, and this component should have as low an alkali content as is available. The higher the alkali content of the cement, the more likely that the cement will cause alkali burns on the skin of the tradesmen who use the product. The Type I Sun Belt, Capitol, and Texas Lehigh Portland Cements are most preferred. Type II and Type V Portland Cements may be used where sulfate resistance is an issue.
Type III Portland Cement is not preferred. Type III Portland Cement produces high Portland strengths but does not impact the speed with which the pozzolans react. Lime liberation by a Portland Cement is roughly related to the gain in compressive strength. As a result, Type III cements can produce an excess of calcium oxide in the mortar before the majority of the reaction water has been absorbed. This can lead to permanent efflorescence.
Type IV Portland Cement can be used, but there are no advantages in using it and may be some disadvantages in its use in cold weather.
Slurries having excess acid such that the pH is less than 7, preferably between 4-6, sufficiently retard the set time for cement compositions. The retarder slurry of the present invention generally includes an acid component, a pH balancing agent, and water.
Accordingly, the acid component of the slurry may include a hydroxycarboxylic acid component which may be any hydroxy-bearing carboxylic acid having one or more carboxylic acid moieties; e.g., it may be a mono-, di-, or tri-carboxylic acid compound. The hydroxy di- and tri-carboxy acids are presently preferred with citric, malic, malonic, glyoxylic and glycolic acids being exemplary. Citric acid is presently the most preferred.
The pH balancing agent of the activator can include any basic alkali and alkaline earth metal hydroxide. Examples of alkali earth metal hydroxides include lithium, sodium, potassium, and rubidium hydroxides while alkaline earth metal hydroxides herein include calcium, strontium, and magnesium hydroxides. Additionally, the pH balancing agent may comprise the salt of any of the above hydroxycarboxylic acid components such as potassium citrate, the potassium salt of citric acid, which is the most preferred.
Traditional retarding components, such as common sugar or metal salts, including borax and gypsum, may be present in the retarder slurry to further control the set time of the composition, although the use of the same is not necessary.
A buffering system can also be employed to help stabilize the retarder slurry or solution. Suitable buffers include Na2HPO4/citric acid solution buffers, and those skilled in the art will readily fashion the required amounts of the Na2HPO4 and citric acid components that should be used to promote stability at a given or requisite desired acidic pH level. The buffering system can be present in an amount of about 1-10 wt %, based on the combined weight of the retarder slurry.
Water is added to the pH balancing agent of the retarder slurry so that the weight percent of the slurry adds up to 100 wt %. Then, the acid component may be added in a certain weight percentage.
Further, with respect to the cement composition, aggregate such as sand (e.g., fine, white, silica, etc.), clay, lime, etc. is used as filler material therein. The aggregate should be non-absorptive, angular and hard with moisture content preferable less than 0.05%. Finally, water is added so that the weight percent of the final cement composition adds up to 100%.
The method for retarding the set time of cement includes providing a retarder slurry with a pH of less than 7, preferably between 4-6. The retarder slurry is formed by combining about 10-15 wt % of a, the pH balancing agent no greater than about 5 wt % of an optional additional retarding component, and water with the foregoing adding up to 100 wt %. Additional 1-12 wt % of an acid component is added to the mixture so that the slurry is acidic.
Fly ash F and C, Portland Cement, and aggregate are provided in varying weight percentages. Preferably about 5-20 wt % for each of the fly ash F and C components, about greater than about 20 wt % of the Portland cement, and about 50-70 wt % of aggregate are provided to equal 100 wt % of the dry mix. With respect to the acidic retarder slurry, about 1-12 wt % of retarder slurry for each 100 parts by weight of the dry cementitious composition then is mixed with the fly ash, Portland cement, and aggregate. Finally, additional water is added to the mixture.